Nonaqueous electrolyte secondary batteries, especially lithium secondary batteries, each of which has a high energy density, have been widely used as batteries for use in, for example, a personal computer, a mobile phone, and a portable information terminal.
Such a nonaqueous electrolyte secondary battery, typified by a lithium secondary battery, may let a large current flow and generate heat in a case where an accident such as a breakage in the battery or in a device using that battery has caused an internal or external short circuit. Thus, between a cathode and an anode of the battery, there is provided a separator that imparts, to the battery, a shutdown function of preventing further heat generation by blocking passage of ions between the cathode and the anode.
In order to achieve an electronic device that is smaller in size and lighter and has a higher capacity, there has recently been a demand for a separator that is filmy and highly strong. Against a background of this, as disclosed in, for example, Patent Literature 1 and Patent Literature 2, there have been proposed various films each of which is made of ultra-high molecular weight polyolefin having a weight average molecular weight of not less than 500,000 and various methods for producing such films. According to these methods, ultra-high molecular weight polyolefin is dissolved in a non-volatile solvent such as liquid paraffin, a gelatinous film, for example is formed from a resultant solution, the gelatinous film containing the non-volatile solvent is partially subjected to an extraction treatment with use of a volatile solvent and then heat-stretched, and thereafter a residual non-volatile solvent is extracted again.
Patent Literature 3 proposes a film that is uniformly deformed during winding of a battery by specifying (i) degrees of orientation of orientation axes 1 and 2 and (ii) and a ratio of an integral value of the orientation axis 1 to an integral value of the entire orientation.
Patent Document 4 discloses a laminated separator obtained by (i) removing, from a sheet made of a polyolefin-based resin composition obtained by molding, by rolling, a resin composition containing ultra-high molecular weight polyolefin, olefin-based wax, and a water-soluble filler, the water-soluble filler with use of an aqueous liquid, and then (ii) coating the sheet with a heat-resistant layer and a porous film that has been twofold to twelvefold stretched and is highly uniform in thickness.